This invention relates to a system for carrying out predictive encoding of an autocorrelated signal into an encoded signal and/or predictive decoding of an encoded signal derived by the predictive encoding from an autocorrelated signal into a reproduction of the last-mentioned autocorrelated signal. An example of the autocorrelated signals to which this invention is applicable is a television broadcast signal.
In a television signal, picture signals have a strong degree of correlation along both horizontal and vertical directions of each picture and along the time axis. It is therefore possible, by resorting to predictive encoding and decoding, to reduce the amount of information to be actually transmitted. According to predictive encoding applied to the picture signals, each picture element is represented by the instantaneous amplitude of a prediction error signal which is the difference between an unprocessed signal for the picture element and a prediction signal derived for an adjacent picture element. In intraframe predictive encoding, such as element-to-element or line-to-line predictive encoding, the prediction signal is derived from a signal for a picture element within the same frame. In interfield or field-to-field predictive encoding, a corresponding picture element in a next previous field to the picture element in question is used for deriving the prediction signal. In interframe predictive encoding, such as frame-to-frame predictive encoding, a picture element used to derive the prediction signal corresponds in a next previous frame to the element in question. Use is also possible of a combined prediction signal obtained by a combination of the above-named modes of predictive encoding and, if desired, other similar modes of predictive encoding. The intraframe predictive encoding provides high efficiency results in the case of pictures which do not have sharp contrast. The interframe predictive encoding shows an excellent efficiency when applied to pictures of still or slightly moving objects. The interfield and combined predictive encoding provides result intermediate the intraframe and interframe predictive encoding.
In a conventional predictive encoder for a television signal, use is generally made of only one of various manners of predictive encoding. This is objectionable for a television broadcast signal in which the scenes being televised may vary slightly and severely at different times, and may sometimes be simple and at other times complicated. In "Conference Record", Volume II, pages 27-12 et seq., of International Conference on Communications held June 16-18, 1975, in San Francisco, California, the United States (IEEE Catalogue No. 75 CH 0971-2 CSCB), a predictive encoder is proposed with reference to FIG. 4 (pages 27-13). The proposed encoder is operable in both modes of intraframe and interframe predictive encoding. Once in each block interval equal to a predetermined number of the picture elements, a mode controller checks which of the modes provides a higher efficiency for the time being and selects the more efficient mode. With this encoder, it is necessary to transmit the information representative of the mode along with the prediction error signal. Also, it is inevitable that the efficiency is seriously adversely affected when the scene being televised varies severely. In any event, the proposed encoder is complicated in structure and is incapable of reducing the total amount of information of transmission by as much as expected.
For the television signal, the three predetermined durations may be any three of the following: an interval between two adjacent picture elements; a horizontal line period; a field period; a frame period; and a like period. It is therefore possible to say for an autocorrelated signal of the type described that one of the predetermined durations is equal at least to the equal spacing between two adjacent ones of the instants and that others of the predetermined durations are longer than the above-mentioned one predetermined duration. The prediction error signal as named herein has an amplitude variable from one of the instants to another in compliance with the autocorrelated signal.